Metal-air cells are well-known, and include a metal fuel electrode and an air electrode. During discharge, the metal fuel is oxidized at the metal fuel electrode and oxygen is reduced at the air electrode. In metal-air cells of the rechargeable (a.k.a. “secondary”) type, the metal fuel may be reduced on the fuel electrode, and oxygen may be evolved by oxidation at the air electrode or a separate charging electrode.
One of the most promising materials for high-performance batteries is aluminum due to its high volumetric capacity (8043 AhL−1) compared to other typical battery materials, such as zinc (5849 AhL−1) and lithium (2061 AhL−1). Additionally, aluminum is both an abundant and relatively inexpensive material. Usually, it is possible to obtain adequate reduction-oxidation (“redox”) behavior of aluminum in a non-aqueous solution using AlCl3 as a source of aluminum ions in chloride ionic liquids and molecular solvents. However, the chloride ions can have significant drawbacks. For example, the main species responsible for the electrochemical behavior of aluminum (Al2Cl7−) is not air stable and easily decomposes in the presence of small quantities of water. The challenges of using aluminum as a battery material are compounded by the extremely rapid formation of an oxide layer on aluminum metal. In a battery, this oxide layer can passivate aluminum electrodes, even in electrolytes with low water concentration. If a constant positive current is applied to an aluminum electrode, the overpotential needed to maintain the current increases with time, thereby drawing an increasing amount of parasitic power.
In order to overcome the inherent challenges presented by the selection of aluminum as an electrode material in a high-performance battery, an electrolytic medium that is able to substantially inhibit the surface passivation of an aluminum electrode and simultaneously allow the redox reactions to take place within the battery is necessary. Specifically, what is needed is:                a. An aluminum chemistry and electrolytic media that is air- and water-stable.        b. An electrolytic media that, under open-circuit conditions, forms a nearly perfect passivating film at the aluminum interface (i.e., does not permit self-discharge to take place at the aluminum electrode).        c. An electrolytic media that, under polarization or discharge conditions, “lites off” (i.e. is removed under electrochemical action) the passivating film at low overpotential and allows sustained faradaic aluminum oxidation to occur (i.e., that enables a power dense, high capacity battery configuration).        d. An electrolytic media that solvates or complexes aluminum ions such that faradaic oxidation results in etching of the aluminum interface as opposed to formation of an anodic oxide film (i.e., that enables flat discharge and high capacity).        e. An electrolytic media that, upon going from an anodic polarization condition to an open-circuit condition, rapidly re-forms the passivating film to prevent self-discharge (i.e., allows for many partial discharge events, and long shelf-life after the first discharge event).        f. An electrolytic media that has a high boiling point and a low melting point so as to efficiently operate in a wide climate spectrum.        g. A system that minimizes parasitic reactions, enabling the highest possible current efficiency.        
Several methods of protecting reactive-metal electrodes (e.g. aluminum) from surface passivation have been proposed. Explicitly protective measures that employ ion-selective membranes, fast ion-conducting ceramic membranes, two-phase electrolytic systems, and thick-variant air cathodes are several non-limiting examples. However, all of these methods result in complex cell architectures that remove the majority of the energy density offered by aluminum.
Rather than pursuing explicit protection of the chemistry of an aluminum-air cell, which substantially reduces available energy density, a solution is provided wherein the aluminum-electrolyte interface is implicitly protected and optionally activated. An electrolytic media that protects the electrode surface by preventing passivation is made possible through the use of solution activators, or dissolved metal salts.